A. Field of the Invention
This invention relates to surface controlled subsurface safety valves utilized to control flow at a subsurface location in a well. More particularly, the safety valve has a single control line and is connectable in and is retrievable with well tubing.
B. Prior Art
When properly installed as part of a subsurface safety system in a well, safety valves are designed to automatically shut-in a well below the earth's surface in the event of erratic changes in flowline pressures, damage to the wellhead or malfunction of surface equipment. Safety valves are designed to be either installed in a tubing string or made up as part of the tubing string. Regardless of the manner in which they are installed in a well installation, they are designed to close on demand from the surface of the well, providing the well operator with complete control of the valve's operation.
Typical of the safety valves available are those illustrated on page 5981 of the Composite Catalog of Oil Field Equipment and Services, Vol. 4, 34th Ed., 1980-81. There is shown a Type DL ball safety valve and a Type QLP flapper safety valve, manufactured and sold by Otis Engineering Corporation. Each of these valves is designed to be connected in and retrievable with the tubing string. For this reason, they are typically referred to as "tubing retrievable" safety valves.
Another feature of the illustrated safety valves is that they are operated by a single pressure fluid line extending from the valve to a source of pressure fluid at the surface of the well. The pressure fluid arriving at the valve enters a variable capacity pressure chamber which expands upon an increase in pressure transmitted to the valve from the surface of the well. The variable capacity pressure chamber is usually housed in an annular area located between the safety valve housing and an inner, longitudinally movable tubular sleeve. This sleeve, when moved downward operates to open a closure device allowing well fluids to flow through the safety valve.
The sleeve noramlly is biased by a spring or some other resilient urging means so that upon release of pressure in the variable capacity pressure chamber, the sleeve is moved back to its first position, allowing the closure device to return to its closed-to-flow position.
Thus, in order to accomplish its "safety" function, the valve must be able to close upon release of pressure in the variable capacity pressure chamber. The pressure chamber is normally isolated from well pressure by O-ring or other types of seals being placed so as to seal off an area in the annular space between the operator sleeve and the safety valve housing. In the event a seal fails, well pressure can enter the pressure chamber. If the well pressure exceeds the spring force required to close the safety valve, the well cannot be closed to flow. Also, well pressure can invade the conduit used to transmit pressure fluid to the valve, causing a blowout through the conduit.